1. Field of the Invention
The present invention relates to a circuit arrangement for operating a plurality of opto-electronic display elements, e.g. luminescent diodes, in particular those which are used in terminal devices of telephone systems for the optical display of switching states and/or operating states, where the display elements form part of a matrix arrangement and can be activated individually or in groups by the connection of items of control information to the particular drive lines.
2. Description of the Prior Art
Specific operating states are displayed by opto-electronic display elements, primarily in the form of so-called luminescent diodes or light emitting diodes, in various ways. As an example of use in telephone systems, reference may be made to use in subscriber stations in which a switching operation can be directly effected by operating different keys. These keys are assigned optical display elements in the form of luminescent diodes. By an appropriate operation of the luminescent diodes, specific operating states or seizure state of the system in question, or the subscriber stations within this system, can be signaled by different luminescent states. If at least one group of these luminescent diodes is arranged in a matrix, they are actuated by the connection of control signals to a specific member of a plurality of column and row lines, which connected is effected by a corresponding control unit. Each of these lines can be assigned an electronic semiconductor switch, e.g. a transistor serving as a control switch which is switched through by the aforementioned items of control information. The operation of the individual luminescent diodes by column lines and row lines within the matrix arrangement can be effected in a multiplex operation. In the drive circuit, this saves storage elements storing the particular required switching state of each luminescent diode.
In uses in which additional electronic components are provided in addition to the aforementioned matrix arrangement, generally only a single voltage source is available which is adapted in respect of its volage value to the supply voltage of these units. Generally, a lower supply voltage is required as a supply voltage for the opto-electronic display elements, e.g. luminescent diodes. In order to be able to use the aforementioned voltage source as an operating voltage, an appropriately dimensioned series resistor is provided. A series resistor of this kind is already required, however, to compensate the tolerances in the current flow resistance of the opto-electronic display elements. A current limiting measure of this kind serves to avoid the harmful influence of a high peak current, determined by the tolerances in the current flow resistor for the particular activated display element. When these display elements are arranged in a matrix, thus a series resistor of this kind should be provided at least for one type of drive lines, e.g. column lines. This series resistor is arranged, for example, in the main current path of the controllable semiconductor switch assigned as a control switch to each such line.
The power loss which occurs across the aforementioned series resistors can provide disturbing in those cases in which the aforementioned opto-electronic display elements are used in devices whose overall feed power is transmitted by way of corresponding feed lines which, depending upon the location of use, differ in line length. This would be the case, for example, when the display elements are arranged in the subscriber stations of a special type of telephone system. It is necessary to keep the overall current requirement of such a terminal device as low as possible. In establishing the voltage value for a central feed voltage source which supplies the feed power for the modules contained in a terminal device of this type, it is necessary to take into account the voltage drop which occurs in the event of a predetermined overall current consumption for the maximum line length. When the terminal device is connected to an extremely short line, a large current requirement of this kind would result in a correspondingly large and, thus, an extremely disadvantageous heat loss.